It's against my better judgment to step into this debate, but, as someone intimately familiar with hostilities in the Middle East, I think I can actually add something here.
The United States specifically has absolutely nothing to do with anything going on in Iran. Let's be very clear about that. While the United States is a key member of the nuclear non-proliferation treaty, and is obligated to help make sure that certain types of regimes do not get their hands on nuclear weapons, the actual basis for the conflict in the Middle East, and the continuation of it, has about as much to do with the United States as the Pope has to do with abortions.
Iran is sort of an interesting place. On one hand, Iranians strive to be accepted by other Islamic nations, and present themselves as a leader in that group. On the other, the Iranians prize modern culture and ideas. The reason that's problematic is that in order to appease potential allies, and stop itself from becoming the target of a religious pogrom by other Islamic nations, Iran continues to hold a fundamentalist government. What that means for the region is that because Iranians are interested in modern ideas, Iran is actually a fairly well to do place in the region, and is one of the few places over there that has the potential to round up the capital to build nuclear weapons without basically destroying the country in the process in the economic sense. Because they have that capability, there's pressure to use that potential to attack Israel, which has long been the target of (and often only means of unifying) the surrounding Islamic countries.
The reason that the United States is involved in the issue of Iran is twofold-- the Iranians aren't exactly fond of the United States, since the United States has meddled in their internal affairs enough to pretty much cement them as uncooperative at best, and downright hostile more often than not. So, naturally, the Iranians having serious military hardware poses a direct threat to the interests of the United States in the region-- only one of which is Israel. The other reason is a bit more political, rather than military-minded-- the United States is the current holder of the largest stockpile of nuclear, chemical, and biological weapons in the world. They haven't been deployed because of the catastrophic damage they can do. The Iranians, however, are just crazy enough to push the little red button, and what that means is that after the Iranians wipe Israel off the map, they're going to go after their rivals in the region-- Iraq and Turkey both come to mind. So, basically, you get into nuclear war. Because of their proximity, Pakistan could potentially become involved, and they're nuclear-capable as well. The reason Pakistan is an issue is that they're Shites, if I recall correctly, and the Iranians are Sunnis. Or maybe it's the other way around-- I can never keep who's what straight, I just know which Islamic nations hate each other on that basis. Either way, Iran and Pakistan are of different Islamic persuasions, so if Iran were to push the button, Pakistan might get involved purely to defend their religious interests.
Basically, Iran having nuclear arms is enough to destabilize the region (and yes, it can be more unstable than it already is-- the Muslims aren't fighting each other outside of Iraq, and even that's sort of slowing down-- it'd be real easy to get that going on a region-wide scale, though.) and that's a serious global concern, because everyone over there is willing to do whatever it takes to win-- and as much as I hate to say it, Israel included (though I tend to think Israel is justified in whatever it does, regardless of what the international community has to say about it.)
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